This invention relates generally to communications and, more particularly, to wireless systems.
It is well known that power control is critical for CDMA (code division multiple access) wireless systems such as those based on the IS-95 standard (e.g., see Holtzman, J. M., xe2x80x9cCDMA Power Control for Wireless Networks,xe2x80x9d in Third Generation Wireless Information Networks, S. Nanda and D. J. Goodman (eds), Kluwer Academic Publishers, Boston, Mass., 1992; and TIA/EIA/IS-95 Interim Standard, Mobile Stationxe2x80x94Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System, Telecommunication Industry Association, July 1993). The ultimate objective of power control in CDMA systems is to achieve a desired speech quality on a particular link at a minimum transmit power level. Without effective power control, the capacity gains expected from a CDMA wireless system will not be realized. This is especially true for the reverse link (uplink) of a CDMA system (i.e., from a mobile station to a base station). Unless the transmit power of the mobile station is tightly controlled, the reverse link will not be able to operate at or near its potential capacity in a cellular communications environment (e.g., fading, etc.).
Realizing the importance of power control for the reverse link, the IS-95 standard has provided for a power control scheme known in the art as xe2x80x9cinner loop power control.xe2x80x9d In this scheme, a base-station transmits a 1-bit feedback signal to a mobile station every 1.25 milliseconds (ms). The 1-bit value of this feedback signal is representative of whether an estimate of the instantaneous signal-to-noise ratio (Eb/N0) of the received signal at the base station (transmitted from the mobile station) exceeds, or falls below, a target signal-to-noise ratio EbT/N0T. Correspondingly, when the mobile station receives this feedback signal, the mobile station raises its transmit power by 1 dB or lowers it by 1 dB depending on the value of the feedback bit. Thus, the inner loop power control scheme provided by the IS-95 standard helps maintain the signal-to-noise ratio of the received signal at the base-station close to the target EbT/N0T.
As noted above, the ultimate objective of a power control scheme in the context of CDMA systems is to achieve a desired speech quality on a particular link at a minimum transmit power level. A simple, quantifiable, measure of the speech quality on a link is the associated frame error rate (FER) on that link. For CDMA systems based on IS-95, the desired speech quality can be said to have been achieved on a link if the FER is at or below a certain level (e.g., 1%). For a given fading environment, the FER is a function of the average Eb/N0 at the receiver. Since, as described above, inner loop power control helps maintain the receiver Eb/N0 close to the target EbT/N0T, the FER is, ultimately, determined by the target EbT/N0T. Therefore, to achieve the desired speech quality in a given fading environment, the target EbT/N0T needs to be set at a level which is appropriate for that environment.
Unfortunately, there is no fixed EbT/N0T target that achieves the desired FER in all fading environments. Therefore, those in the art have developed an adaptive mechanism that adjusts the target EbT/N0T accordingly. This mechanism, referred to hereafter as xe2x80x9cReverse Outer Loop Power Controlxe2x80x9d (ROLPC) monitors the FER and changes the target EbT/N0T depending on whether the FER is below, or above, a desired threshold. By directly using the FER to drive the target EbT/N0T, the current ROLPC achieves its objective very well in reasonably steady fading environment. However, since the FER monitoring process implicit in this technique is rather slow (with time constants of the order of a couple of seconds), its performance can deteriorate in a dynamic environment with rapidly changing fading characteristics.
As such, in order to improve the speed of the ROLPC, the commonly assigned U.S. patent application of Carl Weaver and Wei Peng, entitled xe2x80x9cSymbol Error Based Power Control For Mobile Telecommunication System,xe2x80x9d Ser. No. 08/346800, filed Nov. 30, 1994, describes a symbol error (SE) based technique which potentially improves the performance of ROLPC in a dynamic fading environment. This Fixed SE rate (SER) target ROLPC technique, which is based on the premise that the SER and FER are strongly correlated, tries to maintain the SER close to a pre-determined fixed target SER value. Thus, after every frame the associated symbol error count is compared with the target SER and the EbT/N0T target is raised or lowered depending upon whether the symbol error count was above or below the SER target. The updated EbT/N0T target is used to generate inner loop feedback bits during the next frame.
The above-mentioned Fixed SER target ROLPC technique uses a fixed SER target for the mean value of SER. Notwithstanding the performance improvements possible with the above-mentioned Fixed SER target ROLPC technique, I have observed that the correlation between the SER and FER varies across different cellular communications environments. In fact, for a given (fixed) SER target, the FERs in different fading environments can differ by an order of magnitude. In other words, the above-mentioned Fixed SER target ROLPC technique cannot maintain the FER close to the target in different fading environments. As such, in order to achieve a desired FER, different environments require different SER targets. Therefore, and in accordance with the invention, I present a symbol error count based ROLPC technique with adaptive SER targets. As a result, the inventive concept provides a symbol error (SE) based ROLPC technique that achieves desired FER under different fading conditions.
In an embodiment of the invention, a base station uses a 2nd order statistic, e.g., standard deviation (variance), to identify, or act as a signature of, a particular cellular (wireless) communications environment. The signature is used to set a target symbol error rate appropriate for the current environment. The EbT/N0T target is adjusted as a function of a comparison of the symbol error count with a dynamically adjusted SER target. The base station monitors the standard deviation of the symbol error count of a received signal (transmitted from a mobile station). The EbT/N0T target is adjusted as a function of the value of the standard deviation. The adjusted EbT/N0Ttarget is used to provide power control.
As a result, the inventive concept provides a technique to control FER in a variety of cellular communications environments and, at the same time, keep the performance benefits of a SER based technique.